Do agressive coalescing on this graph.
returns the new graph and the list of pairs of nodes that got coaleced together.
for each pair, the resulting node will have the least key and be second in the pair.

Freeze one node in the graph
This if for the iterative coalescer.
Look for a move related node of low degree and freeze it.

We probably don't need to scan the whole graph looking for the node of absolute
lowest degree. Just sample the first few and choose the one with the lowest
degree out of those. Also, we don't make any distinction between conflicts of different
classes.. this is just a heuristic, after all.

IDEA: freezing a node might free it up for Simplify.. would be good to check for triv
right here, and add it to a worklist if known triv/non-move nodes.